Welcome to Librarium Online!
I've been browsing through the Dark Heresy corebook recently and something struck me as I looked at the tech-priest career path. Electro-priests were a stage on the career path. All the stuff I'd heard up until that point suggested that electro-priests were a separate thing from more general tech-priests, and indeed electro-priests were part of the warrior caste of the AdMech (although admittedly this stuff dates from before Skitarii existed), not a step on the road to becoming a Magos. It also strikes me that a character might also stay a sniper (for example) for years, forgoing any promotions and not getting reassigned, and so they should stay as snipers, regardless of how much xp they accrue. The principle is, I think, more generally applicable to classes in DH. Conversely, I also think it a little weird that if a priest (for example) gets an elite advance in something like Forbidden Lore (dæmonology) before they hit Exorcist (or whatever level allows that), it seems somewhat limiting to say that they couldn't use their new opportunities to further their existing knowledge.
My solution (untested) is to allow characters to stay the same class as they accrue XP if they wish, drawing from the same skills table for as long as they wish, without having to "progress" along the tree. They can of course move on at a later stage, but would have to spend some time in each step (not sure how much XP as yet, this is still a somewhat unformed idea) before moving on again. Maybe just get back on the advancement track where they got off, I don't know. As well as this, the skills in question become open, so where you could only before buy Forbidden Lore (Dæmonlogy) in one class, you could go up to +30 without moving along the career path. This would represent a deeper, but narrow, field of expertise in an area than someone who's just moved into the path and uses it to meet the right people to advance further up in some organisation or other. What do people think?
My other thought is that (unless you assume something like the Vitality of the Star Wars d20 system) the location-based critical damage comes from a centrally determined critical total is a little silly. Having your arm nearly hacked off does not mean that your skull will explode when tapped with a spoon. I'm exaggerating, but I hope you get my point. Would location-specific critical totals make more sense?
There was possibly also something else, but I've forgotten it for now. Any and all thoughts on the ideas I've suggested would be welcome.
I don't think anyone in our group has ever paid attention to the titles for each stage - they are guidelines showing what kind of individual would be the equal of the player's skill in the bigger scheme of things. Regardless of what the player's rank is they are ultimately not a sniper or an electro priest or a magos ... they are an inquisitorial acolyte/henchman/investigator.
It's odd but it does put a very definite line under your health level.
So yes, a rank 2 character can have 4 levels of sound constitution.
It's also worth noting that wealth is part of the 'class' system in the 40k universe - a noble born may be tooling around in power armour while the scum character is scraping for ammunition, but that's his lot in life. The inquisitor (or other characters) are free to pony up extra cash or equipment to help the little guy out - but he'd better be worth the effort, a scum that can't steal what he needs better be damned good at something else.
The skill constraints are partially for game balance, and partially to represent the character's background training, specializations, and talents. You can buy skills outside of your chart but they cost a lot more - representing how difficult it is for say a scribe to learn how to fire a heavy weapon as opposed to a full time professional soldier.